View Full Version : Short stroking question
I bought an upper second hand and it worked great.
It was a Centurion 16" mid length barrel cut down with a pinned and welded compensator to make it 16OAL. It had a Rainier BCG and upper receiver. It ran flawlessly in a carbine class.
I swapped the receiver and BCG for Centurion stuff to make it a single brand upper. I took it to a range, re-zerod it and it seemed fine. I used about 50 rounds.
A week later, Im back in another class and its short stroking. Luckily, I was able to swap uppers and had no interruption of training.
Upper
Former 16" cut to 14.5" Centurion CHF mid length barrel with pinned Battlecomp brake
Centurion receiver
Centurion BCG
Centurion quad rail
Ammo: MEN 56gr 5.56
I tried 2 different lowers that I know work fine with all of my other uppers and the problem wasn't resolved.
They BOTH have Geissele super 42 braided springs and H1 buffers and run well with all of my other uppers.
The gas tube WAS sealed when it was running fine. When I put it back together, I did not seal it. Since I had this issue, I took it back apart and sealed it with blue locktite. Hopefully this works.
My questions are: Could gas tube sealant actually solve this or should I keep digging before I hit the range again? I feel like IF the answer is yes, then there isnt much margin of error preventing this from happening again. Also, I don't normally seal gas tubes into the gas block and don't have issues with other builds. Is it luck? Most others are 16"mids but I do have another 15ish" mid with a pinned and welded brakes that runs fine. This is 14.5" so dwell time is a little shorter than the 16s.
If I go to a carbine buffer, am I just putting a bandaid on this and not identifying or solving the problem? Did it just need to be cleaned before range visits? My round count was low and it only sat for about a week.
Thoughts?
Was the gas block moved/removed during the rebuild?
The Centurion BCG should be GTG, but has it been verified or swapped out to eliminate that variable?
Was the gas block moved/removed during the rebuild?
The Centurion BCG should be GTG, but has it been verified or swapped out to eliminate that variable?
Thanks for asking. I just came back to mention that I did not remove the gas block. That is pinned and sealed so wasnt worth trying to remove.
When I go back, ill bring a few spare BCGs to try.
lonestardiver
12-20-20, 14:26
<edited comment after re-reading OP>
Basic checklist when an AR starts short stroking-
First, check to make sure it locks back when operating the charging handle manually
Second, test your AR using full power 5.56
Third, check for leaks blockages and misaligned parts in the gas system
Fourth, don't waste your time playing with lighter buffers & springs. A properly gassed AR wil be fully functional with a standard spring and an H2 buffer, using full power ammo. Palmetto kits ship with a carbine weight buffer, which is too light
Fifth, when (not if) you start getting empty cases caught in the action, replace the extractor spring with one from Colt or Sprinco- no O ring! Accept no substitutes
I posted this in another thread-
I started having short stroking problems on a high mileage suppressed 11.5" upper.
The BCG related discrepancies were as follows-
- worn gas rings. Installed new gas rings
- leaky gas key. Removed gas key. Cleaned mating surfaces, reinstalled key & staked
- gas tube worn where it fit into the gas key. Installed new gas tube
Dr. Bullseye
12-21-20, 13:42
Looking at MistWolf's post, I would start right here
"They BOTH have Geissele super 42 braided springs and H1 buffers and run well with all of my other uppers."
Go back to the closest thing you can find to mil spec for this rifle and 556 ammo, then test again, then check the rings if still a problem.
Looking at MistWolf's post, I would start right here
"They BOTH have Geissele super 42 braided springs and H1 buffers and run well with all of my other uppers."
Go back to the closest thing you can find to mil spec for this rifle and 556 ammo, then test again, then check the rings if still a problem.
Its an upper that i use across a few different lowers. If i have to build a lower with a carbine buffer, i will. I just don't want to use a carbine buffer to mask the actual problem.
If a dozen uppers with fine across those lowers, then I want this one to also...unless there is a reasonable explanation.
MistWolf says the lighter buffer isnt the answer and these should run H2 buffers. I just want to make sure it works with an H1. That statement also tells me to keep looking.
I really want to know if gas tube leakage at the gas block could make a noticeable difference. I feel like a small amount is normal. Is a sealant there mandatory? Is there a chance that that 1 thing could have been the problem?
Is a 14.5 mid just to soft for an H1 buffer? This is my second and the first was fine so Im leaning toward no.
I may not figure this out until i swap carriers at the range.
Dr. Bullseye
12-22-20, 00:12
You have so many variables. You say you have run this upper with other lowers and then you want to know if it is a gas tube leak issue. Did you have this issue with the other lowers? If not, you have variables in the configuration under discussion that you have to narrow down and check one by one in order to solve the problem. Am I wrong???
You have so many variables. You say you have run this upper with other lowers and then you want to know if it is a gas tube leak issue. Did you have this issue with the other lowers? If not, you have variables in the configuration under discussion that you have to narrow down and check one by one in order to solve the problem. Am I wrong???
It works with neither lower now. It ran fine before swapped receivers and the BCG.
The other lowers work with all of my uppers. The lower configurations are the same from a buffer perspective. I think im gonna have to take it to the range and see if its fixed by sealing that small at the gas tube. If not, ill try a different bcg and see what happens.
I'm not easily keeping track of what you've changed around.
Have you tried dropping butting in a heavier buffer (I hate the Super 42 ftw). Check the gas tube for damage?
I'm not easily keeping track of what you've changed around.
Have you tried dropping butting in a heavier buffer (I hate the Super 42 ftw). Check the gas tube for damage?
The change was the BCG and receiver and at first I didn't seal the tube in the block.
I don't see any damage on the tube.
What don't you like about that buffer? I have a carbine buffer on the way to try, but is that just going to mask a real problem?
It’s not the buffer, it’s the extra power spring some have problems with.
Clearly you changed the BCG and then had failures. Start there. Pretty much always start there, having a known good BCG is key to troubleshooting. Check for any binding in the upper and gas tube alignment as well.
Sealing gas blocks or gas tubes is not a real thing, the tolerance on those parts are among the tightest of anything on an AR. The gas will blow out locktite anyway.
Sealing gas blocks or gas tubes is not a real thing, the tolerance on those parts are among the tightest of anything on an AR. The gas will blow out locktite anyway.
Sealant is actually worse than no sealant because not only are you not contributing sealing, you’re introducing the possibility of crud getting into your gas system that may be counterproductive.
This is the first I'm hearing about sealing gas tubes. Never done it, never heard of it, never had any problems. If you've ever tried to remove one that's well used, you know how self sealing they truly are. After a few thousand rounds they become one with the gas block, and it takes soaking in solvents to ever get them out again.
My best guess would be it's the BCG. Is the gas key on tight? Are there gas rings on the bolt tail?
Are you seeing any strange wear patterns in the upper receiver?
It might help if you posted some pics of the BCG and inside of the upper receiver.
ETA: Something else to check would be making sure the gas system is clear of obstructions. Plug up the chamber and blow into the barrel (preferably with a plastic tube), and see if you feel a strong jet of air on your index finger coming from the gas tube. Then take apart the BCG and blow into the gas key. Sometimes stuff likes to get down in there. Could even be something from the factory like a chip that went down in there. Speaking of which, did you buy this stuff new or used?
Im at the range now shooting 1 round per mag. About 40 cycles in and it locks back every time and elects about 4:00
All i did was pull the tube from the gas block and seal it. I have no explanation.
Im at the range now shooting 1 round per mag. About 40 cycles in and it locks back every time and elects about 4:00
All i did was pull the tube from the gas block and seal it. I have no explanation.
Same ammo?
Same ammo?
Yup. Same mags too.
Maybe it was from sitting a few days and not being cleaned? I cant imagine it being that finicky.
Maybe some debris or something was just in there where it didn't belong and came out when I took it back apart.
Im stumped, but happy that its back to working reliably.
Yup. Same mags too.
Maybe it was from sitting a few days and not being cleaned? I cant imagine it being that finicky.
Maybe some debris or something was just in there where it didn't belong and came out when I took it back apart.
Im stumped, but happy that its back to working reliably.
Yea that's pretty strange. About the only thing I can figure is that there was a little burr somewhere that's worked itself out. Have you checked the inside of the upper receiver for any gouging or anything?
No gouging, but there is obvious phosphate wear on the cam pin and possibly a nick in the receiver forward of the notch for the cam pin.
No gouging, but there is obvious phosphate wear on the cam pin and possibly a nick in the receiver forward of the notch for the charging handle
The nick is normal if I'm understanding you correctly, but maybe post a picture to be sure. If the charging handle is fitting weird that can definitely cause issues. At this point I would suspect some kind of compatibility issue is to blame, which is something you can run into with non milspec parts if you're mixing brands.
The phosphate can definitely cause you to have a little break in period, but it shouldn't be to the point that it's short stroking. If the rifle is so undergassed that a little phosphate stickiness can shut it down then that's too far on the edge for my taste. Especially if it was lubricated.
The nick is normal if I'm understanding you correctly, but maybe post a picture to be sure. If the charging handle is fitting weird that can definitely cause issues. At this point I would suspect some kind of compatibility issue is to blame, which is something you can run into with non milspec parts if you're mixing brands.
The phosphate can definitely cause you to have a little break in period, but it shouldn't be to the point that it's short stroking. If the rifle is so undergassed that a little phosphate stickiness can shut it down then that's too far on the edge for my taste. Especially if it was lubricated.
I think that small mark is normal to. I really suspect that something got in it and came out when I took it back apart.
Nothing is mixed and matched anymore. That was why i took it apart to begin with.
Mixed parts from reputable quality sources are not a concern. At all.
Did you check the brass for any dropped primers?
Mixed parts from reputable quality sources are not a concern. At all.
Did you check the brass for any dropped primers?
I didn't. It was happening in a class so I had to swap uppers and keep it moving.
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